Ligands relative donor capacity

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The comparison of NHCs with various other monodentate ligands such as phosphines and amines on a [MoLsfCOls], a tra/w-[RhL2(CO)X], and various other [M(CO)nLm] complexes shows the significantly increased donor capacity relative to phosphines, even to trialkylphosphines (Table The... 

These effects presumably result from the slightly lowered electron-donor capacity of the phosphetane ligands relative to phospholanes. 

Thulium(II) complexes are stabilized by phospholyl or arsolyl ligands that can be regarded as derived from the cyclopentadienyl group by replacing one CH group by a P or As atom. Their decreased n-donor capacity relative to the parent cyclopentadienyl system enhances the stability of the Tm(II) center, and stable complexes of the bent-sandwiched type have been isolated. 

However, it is sometimes profitable to compare the relative stabilities of ions differing by unit charge when surrounded by similar ligands with similar stereochemistry, as in the case of the Fe3+—Fe2+ potentials (Table 17-1), or with different anions. In these cases, as elsewhere, many factors are usually involved some of these have already been discussed, but they include (a) ionization enthalpies of the metal atoms, (b) ionic radii of the metal ions, (c) electronic structure of the metal ions, (d) the nature of the anions or ligands involved with respect to their polarizability, donor pir- or acceptor d77-bonding capacities, (e) the stereochemistry either in a complex ion or a crystalline lattice, and (f) nature of solvents or other media. In spite of the complexities there are a few trends to be found, namely ... 

The concept of a structure-reactivity relationship implies that changes in structure should be quantitatively reflected in some measurable reactivity parameters associated with the molecule. For metal complexes, the capacity of the ligand structure to influence chemical properties is measurable in terms of reactivity parameters such as stability constants, rates of ligand dissociation, and reduction potentials. The influence of structure on chemical reactivity can often be rationalized in terms of steric and electronic components. MM calculations can be used to quantify the steric components present in the system. For metal complexes with series of ligands in which electronic effects are relatively constant (same number and type of donor atoms), reactivity differences can be attributed primarily to steric effects. In such cases, MM calculations have been used to obtain correlations between structure and reactivity. 

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